1. Field of the Invention
The present invention relates to a black water-based ink composition that can be used favorably in inkjet recording, that has excellent storage stability, and that also has excellent hue, and to an inkjet recording method in which this ink composition is used, and recorded material produced by this recording method.
2. Description of the Related Art
Inkjet recording is a known method in which an ink composition is discharged in the form of droplets from a narrow nozzle, and recording is performed by causing the ink to adhere to the surface of a recording medium in the shape of text or graphics. Inkjet recording methods that have seen practical use include (i) a method in which an electrostrictive device is used to convert electrical signals into mechanical signals, and an ink composition held in a nozzle head portion is intermittently discharged from the nozzle head by these mechanical signals and made to adhere to the surface of a recording medium in the shape of text or graphics, and (ii) a method in which the portion of the nozzle head that is extremely close to the discharge component is rapidly heated so as to generate bubbles in the ink composition held in the nozzle head portion, and the volumetric expansion of these bubbles causes the ink composition to be intermittently discharged from the nozzle head and adhere to the surface of the recording method in the shape of text or graphics.
The ink composition used in inkjet recording is generally a solution obtained by dissolving various dyes in water, an organic solvent, or a mixture of these. These ink compositions need to be safer and afford better printing performance than inks used for fountain pens, ballpoint pens, and other such writing instruments.
Furthermore, in recent years inkjet printers have come to be used in the production of printed materials for advertising, and printed materials produced by an inkjet recording method featuring a water-based ink composition are used not only indoors, but also outdoors. This means that the printed material may be exposed to sunlight and/or light from various light sources, and to chemical substances contained in the atmosphere such as ozone, nitrogen oxides, and sulfur oxides, and must not be degraded by these over time. Various studies into colored inks such as yellow, magenta, and cyan have been conducted in an effort to meet these requirements. Progress has been made with black inks, depending on the dye density and what proportion of the image is accounted for by black ink, because the ink itself is more durable than colored inks, for example. However, black ink is very important from the standpoint of achieving good contrast in an image, and development has been accelerating in recent years along with improvements to the characteristics of colored inks.
In the midst of all this, a disazo dye with the following structure, for example, is known as a dye capable of improving the gas resistance, and particularly the ozone resistance, of an ink composition in which a dye is used as a colorant (see Japanese Laid-Open Patent Application H3-229770 and Japanese Laid-Open Patent Application H3-91576, for example).

However, even an ink composition containing the above-mentioned dye still has inadequate light resistance and ozone resistance, and when its printed material is exposed to light, ozone, and the like, the quality thereof deteriorates over time, resulting in problems such as fading.